Ocean “deserts” expand with global warming

Increased carbon dioxide emissions are not kind to the world’s oceans. It makes them more acidic, potentially dissolving some types of marine life. It makes them warmer, killing coral and sending mobile creatures on a search for cooler water. And now it seems that it’s expanding “deserts” where there’s less life.

Hana Coast. Brent Roraback

Researchers with NOAA (National Oceanic and Atmospheric Administration) are reporting in the journal Geophysical Research Letters that areas of low productivity — i.e. less plant life — increased 15 percent from 1998 and 2007. That’s an increase of 6.6 million square kilometers in the Pacific and Atlantic Oceans. The expansion lines up with an increase in the ocean surface temperature of approximately .02 to .04 degrees Celsius a year.

The science at work is pretty straightforward. As the surface of the water warms, it becomes more stratified (imagine it like a cake — upper layer warm water, lower layer cold water). That prevents the colder, nutrient rich water from mixing with the upper layers. Without the nutrients, there’s nothing to feed algae that help sustain other marine life.

The researchers did offer a caveat as regards the long-term trend, as taken from a NOAA press release:

“The fact that we are seeing an expansion of the ocean’s least productive areas as the subtropical gyres warm is consistent with our understanding of the impact of global warming. But with a nine-year time series, it is difficult to rule out decadal variation,” said Jeffrey J. Polovina, an oceanographer with NOAA’s National Marine Fisheries Service in Honolulu, who authored the study along with NOAA’s Evan A. Howell and Melanie Abecassis of the University of Hawaii.

These barren areas are found in roughly 20 percent of the world’s oceans and are within subtropical gyres — the swirling expanses of water on either side of the equator.

The desert was measured using NASA’s orbiting SeaStar spacecraft and a sensor that measures the density of chlorophyll in phytoplankton, or algae.